Not Applicable.
(copyright) 2002 Electro Scientific Industries, Inc. A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 37 CFR xc2xa71.71(d).
The invention relates to laser micromachining and, in particular, to micromachining layered or multimaterial substrates or devices, such as reinforced or unreinforced epoxy resin, FR4 or green ceramic, with a Q-switched CO2 laser.
Q-switched solid-state lasers are well known and have been demonstrated successfully for many laser micromachining applications. However, micromachining parameters for Q-switched lasers, including their wavelengths (ranging from near infrared to deep ultraviolet), pulsewidths, pulse energies, and pulse repetition rates, have still not been perfected for certain classes of layered organic, inorganic, and metallic microelectronic material constructions with respect to throughput and machining quality, such as cleanness, side wall taper, roundness, and repeatability.
One such class of materials, commonly used in the printed wiring board (PWB) industry, includes glass cloth impregnated with one or more organic polymer resins that is sandwiched between conductive metal layers, typically copper. This material configuration is known as xe2x80x9cFR4 .xe2x80x9d
Another class, commonly used as packaging materials for high-performance integrated circuits, includes unfired, xe2x80x9cgreenxe2x80x9d ceramic materials. These ceramic substrates are formed by high-pressure pressing of powders of common ceramics such as aluminum oxide (Al2O3) or aluminum nitride (AlN). The micron-(xcexcm) or submicron-scale particles are held together with organic xe2x80x9cbindersxe2x80x9d that provide sufficient mechanical integrity for machining operations such as via drilling to be carried out. Afterward, the green material is fired at high temperature, driving off the binders and fusing or sintering the microparticles together into an extremely strong, durable, high-temperature substrate.
U.S. Pat. Nos. 5,593,606 and 5,841,099 of Owen et al. describe techniques and advantages for employing Q-switched UV laser systems to generate laser output pulses within advantageous parameters to form through-hole or blind vias through at least two different types of layers in multilayer devices, including FR4. These patents discuss these devices and the lasers and parameters for machining them. These parameters generally include nonexcimer output pulses having temporal pulse widths of shorter than 100 nanoseconds (ns), spot areas with spot diameters of less than 100 xcexcm, and average intensities or irradiances of greater than 100 milliwatts (mW) over the spot areas at repetition rates of greater than 200 hertz (Hz).
CO2 lasers have also been employed for drilling microvias in multilayered materials, including FR4. Because the 9- to 11-xcexcm wavelength of all CO2 lasers is highly reflected by metals such as copper, it is very difficult to use CO2 lasers to drill substrates that feature overlying metallic layers. Consequently, CO2 lasers are preferably applied to layered substrates with either no overlying copper or an overlying copper layer in which via openings have been previously etched by standard chemical means. Such xe2x80x9cpre-etchedxe2x80x9d or xe2x80x9cconformal maskxe2x80x9d multilayered substrates also typically have no woven reinforcements such as glass fibers in the dielectric layers and are commonly produced in the printed wiring board industry for compatibility with CO2 laser microvia-drilling operations. Common CO2 microvia-drilling lasers include RF-excited lasers, transversely excited atmospheric (TEA) lasers, and fast-axial-flow lasers.
RF-excited CO2 lasers are the most common type of CO2 laser employed for microvia-drilling applications. These lasers employ a radio-frequency (RF) electrical discharge to provide the excitation or xe2x80x9cpumpxe2x80x9d energy that causes stimulated emission from the CO2 molecules. The CO2 molecules, mixed with other gases, are sealed in a tube at pressures well below atmospheric (typically less than 100 torr), and the RF discharge is typically applied across electrodes oriented perpendicularly to the axis of the laser cavity. RF-excited CO2 lasers produce pulses with relatively long pulsewidths, such as 3-50 microseconds (xcexcs), at moderate pulse repetition rates or pulse repetition frequencies (PRFs), such as 2-10 kHz with pulse energies in the 1- to 30-millijoule (mJ) range. The instantaneous power levels of these lasers are low to moderate, typically 1 kW or below, although leading-edge designs are approaching 2 kW.
TEA CO2 lasers employ higher gas pressure (near or above atmospheric) and a DC electrical pump discharge that, as in RF-excited lasers, is applied across electrodes oriented transversely to the laser cavity axis. The main advantage of the TEA CO2 laser design is the short pulsewidth spike and the corresponding high instantaneous power that these lasers can generate. The high power is produced in a 100- to 150-ns spike followed by a low-power xe2x80x9ctailxe2x80x9d lasting up to several microseconds. Pulse energies in the hundreds of millijoules are typical. This combination of pulse energy and pulse spike duration results in extremely high peak instantaneous power, on the order of megawatts. However, because the laser beams exhibit many spatial transverse electromagnetic xe2x80x9cmodes,xe2x80x9d TEA lasers are not highly focusable like the other types of CO2 lasers, so much of the available pulse energy is either masked in the beam-delivery system or by the substrate itself, or both. Nevertheless, TEA lasers can produce vias of excellent quality in FR4. Such vias exhibit glass fiber ends that are cleanly vaporized and flush with the via wall, and little or no over-etching of the surrounding polymer resin. Despite the advantages in via quality, TEA CO2 lasers suffer from the disadvantage of operating at low PRFs, typically 0.2-0.5 kHz. As a result, drilling speed and throughput are limited.
Fast-axial-flow CO2 lasers have seen less application in commercial microvia-drilling applications. Unlike the RF-excited and TEA designs, the tube containing the CO2 gas mixture is not sealed. Rather, the gas mixture flows rapidly through the tube, along the laser cavity axis. Although the gas is collected and recirculated, the need for an external gas reservoir is disadvantageous. Pumping excitation is accomplished by either DC or RF discharge and is usually longitudinal for DC discharge and transverse for RF excitation. A high flow speed is needed to avoid significant heat buildup in the gas while in the discharge region, so this design requires additional gas-pumping equipment that is not needed in the sealed-tube designs.
Despite the added complexity, fast-axial-flow CO2 lasers have become the most common industrial CO2 laser design for applications that require high average power (0.5-10 kW), such as metal cutting or welding. Application to via drilling is limited by the large size and complexity of the laser. Most via-drilling work with these lasers has been aimed toward utilizing the high average power that they generate and obtaining short pulses, such as 1-10 xcexcs, at moderate PRFs through the use of an external modulation device such as a shutter.
For both RF-excited and fast-axial-flow CO2 lasers, the external modulator is required to obtain short pulsewidths (less than several hundred microseconds) needed for microvia-drilling operations. In the externally modulated configuration, the instantaneous power of these lasers is equal to their average power, which is relatively low.
On the other hand, Q-switched CO2 lasers have been used for some time in military imaging radars but have not been applied to material processing until recently. In xe2x80x9cA High-Power Q-switched CO2 Laser and Its Application to Organic Materials Processing,xe2x80x9d Proceedings of the International Congress on Applications of Lasers and Electro-Optics, Sakai, T., Imai, H., and Minamida, K. (1998), the authors report using a CO2 laser design that employed a high-speed rotating chopper wheel to provide intracavity modulation. The basic resonator consisted of a fast-axial-flow RF-excited cavity capable of 2 kW continuous-wave (CW) output in TEM01 mode, which is not highly focusable. The eight sections comprising the discharge region of this laser are quite large, producing a beam 18 mm in diameter. The RF power supply, capable of continuous pumping at 15 kW power level, is also quite large.
Sakai et al. used this laser to ablate films of polyethylene terephthalate (PET) and polypropylene (PP) and found ablation quality similar to that achieved with a TEA laser. The article concludes that xe2x80x9c[a] Q-switched CO2 laser will be an attractive tool for the industrial applications of polymer ablation such as the paint stripping or machining of the microelectronics device.xe2x80x9d
An object of the present invention is, therefore, to provide a high-pulse-energy, Q-switched CO2 laser and/or method for micromachining microelectronics manufacturing materials.
A process of particular interest is the drilling of small holes or microvias in multilayered, laminated substrates such as those used in high-density printed wiring boards and integrated circuit (IC) chip packages. FR4, for example, may be difficult to laser-machine for several reasons. First, the material is highly heterogeneous, particularly with respect to properties governing laser ablation characteristics such as melting and vaporization temperatures. Specifically, the vaporization temperatures of the woven glass reinforcement and the polymer resin matrix differ greatly. Pure silicon dioxide (SiO2) has melting and vaporization temperatures of 1,970 Kelvin (K) and 2,503 K, respectively, while typical organic resins such as epoxies vaporize at much lower temperatures, on the order of 500-700K. This disparity makes it difficult to laser-ablate the glass component while avoiding ablation of too much of the resin surrounding individual glass fibers or in regions directly adjacent to fiber bundles.
Second, most FR4 glass cloth is woven from bundles or xe2x80x9cyarnsxe2x80x9d of individual glass filaments. Filaments are typically 4 to 7 xcexcm in diameter, and yarns range from about 50 xcexcm to several hundred microns in diameter. The yarns are generally woven in an open-weave pattern, resulting in areas of high glass density where yarns cross each other and areas of low or zero glass density, such as between adjacent bundles. Because the locations of vias cannot be selected a priori with respect to the weave pattern, the desirable via locations will vary in glass density. Thus laser micromachining process parameters that work equally well in both high- and low-glass-density regions of the substrate are desirable. Process conditions that are xe2x80x9caggressivexe2x80x9d enough to cleanly vaporize all the glass in high-density regions and at the same time are xe2x80x9cmildxe2x80x9d enough to avoid over-etching or removing too much resin in low-density regions have been difficult to achieve with existing CO2 laser systems.
Laser-machining green ceramics poses concerns similar to those for processing FR4 due to the differences in the thermal properties of the organic binders and the ceramic microparticles. The disparity between the vaporization temperature of the binder (again, on the order of 500 K) and the ceramic (3,800 K for Al2O3) influences the way material is removed during laser drilling. Because ceramic has a high vaporization temperature, it is quite difficult to remove green ceramic through direct melting (at 2,340 K for Al2O3) or vaporization of the microparticles.
The preferred laser micromachining process instead relies upon the explosive vaporization of the binder material holding the micro-particles together. When exposed to laser pulses, the binder vaporizes much more easily than the ceramic, and the organic vapor is driven to high temperature at extremely high heating rates, creating localized high-pressure gas regions in the spaces between microparticles. The high-pressure gas then expands rapidly, disintegrating the green material. Thus the green ceramic material can be removed while in its solid state with each laser pulse, at removal rates much higher than could be obtained by its direct vaporization.
Material removal by explosive vaporization of the binder can be either advantageous or disadvantageous in laser-micromachining green ceramics. If the organic vapor pressure is too high or spread across too wide an area, undesirable effects such as chipping or microcracking can occur. If the high-pressure regions are too localized or not hot enough, poor material-removal rates are achieved.
In a preferred embodiment of the Q-switched CO2 laser employed in the present invention, an SPL-100-HE laser produces pulses sharing some of the characteristics of the TEA CO2 laser but is capable of delivering the pulses at much higher PRFs, such as 20-140 kHz. The laser output features a short high-power spike of about 80-150 ns, followed by a lower-power tail, the duration of which is adjustable between about 0.05 and 12.0 xcexcs. Pulse energies ranging from less than 1 mJ to greater than 20 mJ can be attained by varying the duration of the tail. Peak instantaneous power in the spike can be as high as, or higher than, 40-50 kW; while in the tail, the instantaneous power decreases slowly from about 1-2 kW to less than 1 kW over about 5-10 xcexcs.
The laser output characteristics provide unique combinations of laser wavelength, pulse energy, pulse peak power, PRF, and pulsewidth (pulse duration) that are not available in any conventional laser micromachining system. The combination of high peak power, high pulse energy, very high PRF, and variable pulsewidth permits a great deal of flexibility in the manner in which laser energy is deposited in the target material and permits the target material to be removed with a degree of precision or xe2x80x9ccleannessxe2x80x9d and at high speeds previously unattainable with conventional CO2 laser systems.
Due to the unique combination of pulse output characteristics generated by the high-energy Q-switched CO2 laser and the flexibility in tuning individual pulses, this laser is advantageous for solving the problem of drilling microvias in difficult substrates such as FR4 or green ceramics. The heating rates of the organic binder, interstitial pressures, and the duration of each high-pressure event can be controlled by pulsewidth and the rates of energy deposition into, and heat transfer from, the substrate material. The instantaneous power delivered to the substrate throughout the laser pulse or from pulse to pulse in a multipulse burst can be used to govern these effects. Because this laser""s pulses may be tuned for pulsewidth, peak instantaneous power attained during each pulse, and total pulse energy, the Q-switched laser""s output characteristics offer a high degree of control of energy deposition and material-removal processes. This versatility offers the potential for optimizing both micromachining quality (elimination of undesirable effects such as chipping or cracking) and drilling speed.
Other laser micromachining processes in which the speed of material removal and quality (precision) of micromachining are both important may also benefit.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings.